In this paper we review the relationships between artificial intelligence (AI) and climate change. Climate change is a serious threat to the environment and human health, where the main contributing element is the release of greenhouse gases into the atmosphere as a result of human activity. We found that AI is playing two contrasting roles in the climate crisis. Firstly, as a villain that is increasing emissions of greenhouse gases. Secondly, as a hero that supports efforts to reduce emissions. While increasing use of large AI models in data centres increase emissions, improved algorithms reduce the computation required and hence the associated emissions. AI can also be used to increase the energy efficiency of data centres and to improve the productivity of renewable energy sources. This paper presents ways in which researchers in AI can prioritise their work to address the climate crisis and offers some suggestions for policy makers.